DRAFT: implement sequence

src/sequence/flatMapIndexed.ts

@@ -0,0 +1,6 @@
+export function* flatMapIndexed<T, R>(value: Iterable<T>, transform: (index: number, it: T) => Iterable<R>): Iterable<R> {
+    let index = 0;
+    for (const it of value) {
+        yield* transform(index++, it);
+    }
+}